1. Field of the Invention
The present invention relates to a compact vibratory flowmeter, and more particularly, to a compact vibratory flowmeter for measuring flow characteristics of a multi-phase flow material.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
One difficulty in using a flowmeter to measure a flow material is when the flow material is non-uniform, such as in a multi-phase flow condition. In a multi-phase flow condition, the flow material includes two or more of a gas phase, a liquid phase, and a solid phase. For example, a common flow measurement scenario is where the flow material includes gas entrained in a liquid. Air is a commonly entrained gas. Because gas is compressible, the properties of the flow material can vary and therefore the entrained gas can cause erroneous readings in the flowmeter. Entrained gas can degrade the accuracy of mass flow rate and density measurements, and therefore can indirectly affect a volume measurement.
FIG. 1 shows a U-shaped vibratory flowmeter of the prior art. This prior art U-shaped vibratory flowmeter has a very low aspect ratio, where the aspect ratio comprises a meter overall length (L) divided by a meter overall height (H), i.e., the aspect ratio=L/H. It can be seen from this figure that the prior art aspect ratio is typically much less than one, especially for a prior art U-shaped flowmeter. In applications where the conduit diameter is large, it can be seen that the small aspect ratio of this prior art flowmeter will require a large amount of vertical physical space for installation.
In many settings, the physical space that is available for a flowmeter is limited. For example, both the meter overall length (L) and the meter overall height (H) may be dictated by the available installation space. Consequently, there is a need for a compact flowmeter that features both a reduced length (L) and a reduced height (H), and a high aspect ratio (L/H) (i.e., is compact). Furthermore, there is an increasing demand for smaller, more compact flowmeters that can provide a needed measurement capability and a high level of measurement accuracy and reliability.
In the prior art, attempts to produce a compact vibratory flowmeter have comprised scaling down existing flowmeters for such applications and/or using bowed or straight flow conduits. However, this has been met with unexpected complications and with unsatisfactory flowmeter accuracy. One result of scaling down an existing flowmeter design is that the stiffness of a flow conduit can increase by a large amount. This increased stiffness characteristic results in an increase in the drive frequency of the flowmeter. Problematically, this relatively high drive frequency results in a degraded performance and/or accuracy for multi-phase flow materials. When gas is entrained in the flow material (such as air bubbles, for example), the flow material resonant frequency is affected and is lower than a resonant frequency of a pure fluid flow material. As a result, a drive frequency employed by a prior art flowmeter may be at or near a flow material resonant frequency. It has been found through research that the accuracy of a flowmeter decreases as the flow material resonant frequency of a multi-phase flow approaches the drive frequency of the flowmeter. Consequently, the entrained gas results in an inability of the flowmeter to accurately measure flow characteristics of the flow material and to measure non-flow characteristics.